I haven't used either, but http://sparkleshare.org/ http://sparkleshare.org/ and http://owncloud.org/ http://owncloud.org/ seem to be the best open source optio

I haven't used either, but http://sparkleshare.org/ http://sparkleshare.org/ and http://owncloud.org/ http://owncloud.org/ seem to be the best open source options. I may give them a try soon - I'm getting more and more concerned by Dropbox.